Modern drives of magnetic or optical data carriers customarily use brushless direct current motors to drive the spindle carrying the data carrier, or to drive the transport unit for the magnetic tape or other data carrier.
An example of such a data carrier system is a disk drive, which uses floating read/write heads that start off and land on the disk surface. This means that the read/write heads and the disks go through critical phases of friction during which there is a high risk of damage or destruction of the data carrier. There is also a risk of damage to the read/write heads. It is accordingly desirable that the start off and landing phases be as short as possible. However, this requires that the motor have a high starting torque and stopping, or breaking, torque.
The motor size and the rotational speed, however, can not be raised arbitrarily. For a brushless direct current motor generally, EQU K.sub.m =K.sub.v.sup.2 /R (1)
where K.sub.m is the motor constant, which is give by the motor construction, the available motor space, and the magnetic material. K.sub.v is the back EMF constant, also known as the counter voltage constant, and R is the ohmic motor resistance.
The equivalent circuit diagram of a motor consists of a resistance, an inductance, and a generator which generates the counter voltage in dependence on the rotational speed. The supply voltage is divided across voltage drops at the switching transistors, the leads, and across the internal motor resistance, as well as across the counter voltage. K.sub.v is simultaneously, in the metric system while leaving out bearing friction, the torque constant K.sub.t, which gives the torque per ampere generated by the motor. The possible counter voltage divided by the rotational speed is accordingly proportional to the turning moment T. EQU T=K.sub.v /.omega. (2) EQU or EQU T=K.sub.v *I.sub.m ( 3)
with I.sub.m as the motor current.
If the ohmic resistance R is raised, i.e. more windings are wound on to the stator, then the induced voltage rises, as does the back EMF constant K.sub.v. The applied motor voltage divided by the motor resistance gives the theoretical maximum possible starting current I.sub.m.
The supply voltage and the desired rotational speed give the maximum possible value for the rotational moment T, and thereby the maximum possible motor starting torque T.sub.s. However, the torque T must be chosen so that the counter voltage at a given rotational speed still lies below the supply voltage. Therefore, since the ohmic motor resistance is fixed, a motor design must compromise between a high motor starting torque T.sub.s and a large counter voltage.
In the known brushless direct current motors, a fixed supply voltage is used and the motor current is adjusted by means of a speed control loop responsive to the operating conditions of the motor. The motor current control is effected by pulse width modulation and/or series control. With pulse width modulation, the full supply voltage is applied in a pulsed fashion to the motor. With series control, the power supplied to the motor can be controlled by converting some power into lost heat by using a voltage drop across a series resistance.
In the startup phase of the motor, there is no counter voltage present since the rotational speed is zero. For this reason, the starting current must be limited, whereby the starting torque is reduced to relatively low values.
The motor coils, normally numbering three to six, are customarily connected in fixed fashion with the current control circuit and are connected together in a triangular or star shape. The star arrangement is used for unipolar controls, in which all currents flow to a common earth. The triangular arrangement is used for bipolar controls, in which the current in each coil flows in both directions. A star arrangement may also be used for bipolar control if each branch of the star contains two coils in series.
In known current control circuits, there is the possibility of achieving a high starting torque at a predetermined rotational speed and supply voltage by switching over from bipolar to unipolar drive. During startup, the motor is run in bipolar drive, with two coils switched in series in order to achieve a high starting torque. After achieving a given speed, the motor is switched over to unipolar drive. This then has a lower counter voltage at a given rotational speed as a result.
For the supply of energy to disk drives, a supply voltage of 12 volts is normally fixed by industry standard. This substantially limits the power which can be delivered by the direct current motor. If, for example, the motor resistance is 1 ohm and the motor current can be at most 4 amperes, then the maximum power which can be delivered by the motor is given by EQU P=V*I=I.sup.2 /R=16watts (4)
To achieve the maximum possible starting torque, the motor current must therefore be raised. However, increasing the motor current requires a more powerful voltage supply and a motor circuit capable of carrying the additional current. This would result in additional costs.
A motor bridge, consisting of a series of switches, preferably bridge transistors, is normally used for commutation of the motor. The operation of these switches is matched to the commutation phases of the motor. If the inductance of the coil of the stator is subjected to a pulsed voltage, then excess current oscillations arise when the voltage is switched on, and strong excess voltage spikes arise when the voltage is switched off. These oscillations and spikes are minimized in customary circuits by a delayed voltage rise or fall, i.e. by so-called soft switching. The disadvantage of soft switching is an increased power loss in the bridge transistors. If the transistors are hard switched, then so-called snubber networks, preferably RC members, are normally used, which convert the voltage spikes into waste power. However, the remaining current overshoot, caused when switching on an inductance, is one of the sources of motor noise.